Typically the manus and pes both have five digits (pentedactylate). The first digit of the manus is commonly called the =pollex=, and the first digit of the pes the =hallux=. In a very simple =carpus= such as that of _Chelydra_, there are nine bones. They are arranged in a proximal row of three, the radiale, intermedium, and ulnare,--the first being on the radial side of the limb, and a distal row of five called respectively carpale 1, 2, 3, 4, 5, beginning on the radial side. Between these two rows is a single bone the centrale, or there may be two. Similarly there are nine bones in a simple =tarsus= such as that of _Salamandra_. They form a proximal row of three, the tibiale, intermedium and fibulare, and a distal row of five, called respectively tarsale 1, 2, 3, 4, 5, beginning on the tibial side. Between the two rows there is a centrale as in the carpus, or there may be two. * * * * * The following names derived from human anatomy are commonly applied to the various carpal and tarsal bones: =Carpus.= radiale = scaphoid intermedium = lunar ulnare = cuneiform centrale = central carpale 1 = trapezium " 2 = trapezoid " 3 = magnum " 4 } = unciform " 5 } =Tarsus.= tibiale } intermedium } astragalus fibulare = calcaneum centrale = navicular tarsale 1 = internal cuneiform " 2 = middle " " 3 = external " " 4 } = cuboid " 5 } NOTE. The above is the view commonly accepted concerning the homology of the carpal and tarsal bones. But with regard to the proximal row of tarsal bones there is difference of opinion. All anatomists are agreed that the calcaneum is the fibulare and that the intermedium is contained in the astragalus, but while the majority regard the astragalus as the fused tibiale and intermedium, Baur considers that a small bone found on the tibial side of the tarsus in _Procavia_, many Rodents, Insectivores, and the male _Ornithorhynchus_, is the vestigial tibiale, and regards the astragalus as the intermedium alone[13]. He also considers that the mammalian scaphoid represents a centrale. MODIFICATIONS IN THE POSITIONS OF THE LIMBS[14]. In their primitive position the limbs are straight and are extended parallel to one another at right angles to the axis of the trunk. Each limb then has a dorsal surface, a ventral surface, an anterior or =pre-axial= edge, and a posterior or =postaxial= edge. In the anterior limb the radius and the pollex are pre-axial, the ulna and the fifth finger are postaxial. In the posterior limb the tibia and the hallux are pre-axial, the fibula and the fifth toe are postaxial. The Cetacea and various extinct reptiles, such as _Ichthyosaurus_ and _Plesiosaurus_, have their limbs in practically this primitive position.